How precisely repeat shapes?
Model the circle
Add cylinder (&#8679; ShiftA), then enter edit mode (&#8633; Tab) and select everything (A)
Scale the cylinder (S) so it has the same radius as desired for the holes
Move it over on the X or Y axis so it intersects the circle (GX)
Exit edit mode (&#8633; Tab)
Add an empty (&#8679; ShiftA) and rotate it 15&deg; on the Z (RZ15)
Add an array modifier to the cylinder
Disable relative offset and enable object offset
Specify the empty as the offset object in the array modifier
Note: You can press E while hovering over the object selection box to pick an object by clicking in the 3D view
Increase the count until there are cylinders all around the circle
Add a boolean modifier to the circle
Specify the cylinder object as the boolean target 
Set the boolean operation to difference
Apply the boolean modifier
Delete now unneeded cylinder object and empty